The subthalamic nucleus is a key structure in the functional circuitry of the basal ganglia. One of its major roles is to exert powerful excitatory effects upon basal ganglia output neurons in the internal segment of the globus pallidus. It is been proposed that the hyperactivity of this connection underlies most of the motor deficits in Parkinson's disease. Anatomical data obtained during the past year in our laboratory encouraged us to reconsider this hypothesis. Using highly sophisticated tract-tracing techniques at the light- and electron-microscopic level, we showed that the subthalamic nucleus is in a position to exert not only excitatory, but also inhibitory effects upon basal ganglia output neurons. These findings, which have now been supported by other laboratories, necessitate a revision of our current view of the functional circuitry of the basal ganglia in normal and pathological conditions.